This invention relates to a grinding mill in which a cylindrical rotor floats concentrically within a cylindrical vessel in a vertical orientation. The side walls of the rotor and vessel define between them an annular gap or space within which feed particles are comminuted by forceful interaction with particles of a grinding medium. The particles to be ground are introduced in fluidized form and forcefully interact with and contact the grinding medium to reduce their particle size.
A great number of conventional types of ball and wet grinding mills are now in use. In addition to the older forms in which the agitating shaft has stirring elements in the form of rods disposed transversely of the axis of the shaft and spaced a distance therefrom, there are also ball mills in which the agitating shaft carries disk-shaped stirring elements distributed throughout its height. In a recent type known as annular compartment mill, the agitator has a stirring shaft having the form of a hollow cylinder. In addition to the stirring rods mounted on the shell of the hollow cylinder, the grinding compartment also may have on its inside wall rods that project into the annular grinding compartment between the rows of rods distributed throughout the height of the agitator (German Pat. No. 1,233,237). See also U.S. Pat. Nos. 3,149,789 and 3,185,398.
Regardless of whether the stirring elements of the wet grinding or ball mills are constructed as rods or disks, it is still an open question as to whether the reduction in size is accomplished by impact and by attrition between the grinding media, by attrition alone, or by extremly high shearing strain of the carrier liquid. The ball and wet grinding mills of known construction also exhibit in the ground material a broad particle size distribution. Therefore, in the majority of cases where the ground material should have a narrow particle size distribution, i.e. minimal parts of coarse and overly finely grounded materials, the ball mills or wet mills of known construction do not product satisfactory results.
German Auslegeschrift DT-AS No. 1,184,188 describes a ball mill in which a smooth-walled cylinder rotates concentrically or eccentrically within a vessel to grind particles in the gap between the rotor and vessel which are pumped through the gap in an aqueous slurry under differential pressure from an external pump. The structure is somewhat similar to that of the ball mill described in U.S. Pat. No. 3,423,032, which has a restriction at the bottom of the annular gap, small enough to prevent the grinding balls from dropping through it. There is, however, no retainer at the bottom of the annular gap in the mill described in DT-AS No, 1,184,188 and its external pump must generate enouth velocity to pass all components of the slurry upwardly through the gap. This necessitates relatively high pumping pressure and there is still no assurance that the particles will not drop backwardly through the gap and lodge under the rotor.